The present invention relates to electrical measurement devices and, more particularly, to devices for measuring an electric current passing through a conductor.
Many electronic applications such as, for example, electronic metering of electric power and energy usage, require means for producing a signal proportional to an electric current in a conductor. Conventional current measurement devices employ a current transformer having a primary winding in series with the current to be measured. A resistor in series with a secondary winding of the current transformer produces a voltage having an amplitude proportional to the current in the primary winding. This voltage is then employed as a measure of the primary current.
Conventional current measurement devices suffer from a number of drawbacks. Current transformers are expensive devices and, when employed in high-voltage power sources, require suitable insulation thus further adding to cost. In addition, an output current of a current transformer must be scaled to the current-handling capability of the circuit receiving it. Electronic measurement devices such as, for example, electronic watthour meters, are capable of handling only a few milliamperes of current. Load currents, in contrast, may be several hundred amperes. For example, if a single primary turn is employed in such a current transformer, on the order of 100,000 secondary turns are required to produce a full-scale current in the presence of a load current of, for example, 300 amperes. Conventional toroidal transformer cores do not provide sufficient room for this many turns in a device of practical size. In addition, such current transformers require a low value of burden resistance to function with adequate accuracy. A winding with such a large number of turns has an inherently high resistance, thus precluding operation with the required accuracy.
The prior art contains several approaches for dividing a load current in order to produce a sample current or voltage proportional to the load current. A shunt technique, disclosed in U.S. Pat. Nos. 4,182,982 and 4,492,919, splits the current in a conductor between a main shunt path and a parallel auxiliary path. The auxiliary path contains a much smaller cross section than does the main shunt path and current through the combination divides in substantially the ratio of the cross sections. A magnetic core with a winding of many turns is disposed about the auxiliary path. The auxiliary path thus forms a one-turn primary and the many turns about it form a secondary. A current through the secondary is proportional to the current in the primary divided by the number of turns in the secondary. This technique suffers reduced accuracy from the substantial thermal coefficient of resistance of copper which may be as much as 30 percent over the environmental temperature range to which watthour meters are exposed. In addition, it is difficult to obtain a sufficient current division to give the four orders of magnitude reduction in output current compared to load current. Finally, this technique is subject to errors resulting from magnetic flux about the current-carrying conductor making up the shunt path.
A further technique, disclosed in U.S. Pat. No. 4,496,932, employs two slits in a conductor to produce a measurement conductor between a pair of shunt conductors. The measurement conductor is deflected, first in one direction, and then in the other, to provide space for the passage of a one-turn loop of magnetic core material therethrough. In one embodiment, the shunts and the measurement conductor are folded to align holes in two halves thereof. The one-turn loop of magnetic core material is then passed through the aligned holes for receiving a sample of leakage current produced by the presence of the slits and the measurement conductor. A secondary winding of many turns on the core loop provides an output. This device suffers from the presence of strong magnetic fields in its vicinity which are capable of saturating the core and thus introducing errors or cancelling its output. In addition, no provision is provided for cancelling the effects of non-uniform magnetic fields originating external to the measurement device, as are routinely experienced in watthour meters.
Other special problems are encountered in watthour meters. The external configuration, including the positions of connector blades, is established by rigid industry standards. Such industry standards also require that crosstalk between adjacent phases be held to a very low level.